In the manufacture of plastic containers, it is conventional to injection mold or compression mold a container preform having a body and a finish with one or more external threads. The finish typically is molded to its final geometry, while the body of the preform is subsequently blow molded to the desired geometry of the container body. The preform may be of monolayer construction, or may be of multilayer construction in which one or more intermediate layers in the preform body may or may not extend into the finish area of the preform. U.S. Pat. Nos. 4,609,516, 4,710,118 and 4,954,376 illustrate injection molding of multilayer container preforms.
Molding the finish portion of the container as part of the preform presents a number of problems. For example, when the preforms are formed by injection molding, the plastic material typically is injected into a mold cavity at the closed end of the preform body, so that the material must flow along the sides of the preform cavity into the area in which the finish is molded. The finish typically requires more accurate and stable dimensioning than the body of the preform, which may limit the cycle time of the molding process. Furthermore, the finish portion of the preform is of the same material as at least the outer layers of the preform body, which limits the ability to obtain the most desirable characteristics at the finish. When the preform is of polyester construction, such as polyethylene terephthalate (PET), the finish portion of the preform can be wholly or partially crystallized to improve the operating characteristics of the finish area, particularly in hot-fill container applications. However, the requirement that the finish be of the same material as at least the outer layers of the preform body still limits the design capabilities of preform manufacture.
A preform assembly for blow molding a container in accordance with a first aspect of the present invention includes a molded plastic preform having a body and a neck with an external surface. At least one circumferential channel is molded into the external surface of the preform neck for removing the preform from the forming mold. A plastic finish ring separate from the preform is externally secured to the preform neck over the external surface of the neck. A method of making a preform assembly for blow molding a container in accordance with a second aspect of the invention includes molding a preform having a body and a cylindrical neck with an external surface, and at least one circumferential channel molded into the external surface of the neck. A plastic finish ring is secured over the external surface of the cylindrical neck.